The disclosed invention is directed generally to computer storage digital audio tape (DAT) drives, and is directed more particularly to techniques for monitoring read error rates to provide early indications of degrading tape or dirty tape heads before an unrecoverable error occurs.
Digital audio tape (DAT) technology, which was developed for audio programming as defined in the DAT Conference Standard, DIGITAL AUDIO TAPERECORDER, June 1987, published by the Electronic Industries Association of Japan, has been adapted for storage of computer data. An example of a format for the storage of computer data utilizing DAT technology is the American National Standards Instituted (ANSI) Digital Data Storage (DDS) standard, presently in draft form ("PROPOSED AMERICAN NATIONAL STANDARD HELICAL-SCAN DIGITAL COMPUTER TAPE CARTRIDGE 381 mm (0.150 in) FOR INFORMATION INTERCHANGE," ASC X3 Project No. 668-D).
The characteristics that have made DAT technology attractive for computer data storage include high capacity, high transfer rate capability, relatively small media size and low media cost, and the adaptability of DAT technology to conform with personal computer storage device form factors including the 51/4 and 31/2 inch forms.
The use of DAT technology for storage of computer data can be achieved by application of a data storage format, such as the above-referenced ANSI DDS standard, to the DAT Conference audio standard. In particular, the DAT audio standard physical track format is retained, but the contents of the information stored in the tracks is in accordance with the computer data storage format.
The storage media in DAT technology is magnetic tape that is moved relative to tape heads, tape guides, and so forth, and like most tapes is subject to wear with increased use. A worn tape is unreliable and should be replaced. A consideration with tape wear and deterioration is determining when a tape should be replaced. Ideally, a tape should be replaced before tape wear causes difficulties in writing and/or retrieving data from the tape.
One attempt to provide information indicative of tape deterioration has been the DDS format definition of a System Log area on tape. The DDS format provides for three levels of error correction which are referenced as C1, C2, C3, with C3 being the highest level of error correction, and the System Log area was defined for recording of the number of C3 errors and rewrite errors which occurred during the current use of the cassette. The intent was to provide an indication of tape deterioration as manifested by a significant increase in errors relative to the prior use, and some computer storage DAT drives include the capability to indicate a warning if current use indicates significantly more errors than prior use. However, a warning based on the System Log information as to C3 errors and rewrite errors may be too late to prevent unrecoverable errors. C3 read errors occur only when C1 and C2 processing fail to correct a track (i.e., when there are more than 24 C1 errors in a track). If both C1 and C2 fail to correct, it is likely that many tracks in the vicinity of a defect will be uncorrectable, such that C3 cannot recover the data (C3 corrects only two tracks per group). While the increase in C3 errors shows that a head clog or unrecoverable error occurred, such increase does not provide an effective early warning of a head clog or gradual tape deterioration. The same is true of monitoring the number of rewrites during writing. Rewrites are normally done only if a severe tape defect is detected, and thus do not provide an early warning of a head clog or gradual tape deterioration.